The invention relates generally to heat sensitive coatings. More specifically, the invention relates to heat sensitive coatings which do not require a protective coating.
Generally, heat sensitive recording materials include a color producing or heat sensitive layer coated onto a substrate with a protective layer coated over the heat sensitive coating to protect it from exposure to water and/or oils.
Typical heat sensitive coatings are aqueous dispersions of a color former, a color developer, an optional organic sensitizer and a water soluble polymeric binder. The polymeric binder, e.g., polyvinyl alcohol, polyvinylpyrrolidone, or the like, typically is highly soluble in water. Accordingly, heat sensitive layers which contain water soluble polymers, like polyvinyl alcohol, have a high affinity for water such that the resulting recording material may be easily damaged by the presence of moisture. In addition, organic sensitizers and bisphenol-type developers in typical heat sensitive coatings often arc soluble in oil such that contact between the recording material and an oil may result in reversal of an image formed by development at the area of contact.
Protective coatings generally include water soluble polymer binders such as, for example, polyvinyl alcohol crosslinked with a crosslinker such as, for example, melamine-formaldehyde. These top coat layers tend to reduce the incidence of damage to the recording material by water and/or oil, however, they do not protect the recording material from prolonged exposure to water.
The use of a protective coating also increases the cost of the heat sensitive recording material, for example, the costs of the protective coating materials and the processing costs involved in preparing and coating the protective coating are added to the overall costs of the commercial product. Moreover, protective coatings typically reduce the tint strength of the heat sensitive layer due to increased spacing between the thermal head of the thermal printer and the color producing layer when printing. Additionally, protective layers typically include a plastic binder and/or fillers comprising pigment. Plastic binders typically are heat insulating which further reduces the potential tint strength and pigments which tend to mask the developed color of the heat sensitive layer.
A heat sensitive coating has been developed which is environmentally resistant. That is, the heat sensitive coating is resistant to water, oil, cleaning solutions and plasticizers, as well as other environmental stresses including scuffing and scratching, thus obviating the necessity and expense of a protective coating. The heat sensitive coating and recording materials of the present invention may be used with a variety of printers and processes, including, but not limited to, offset and flexo printing.
An environmentally resistant heat sensitive coating of the present invention includes an acrylate polymer of the formula: 
wherein R1 is a hydrogen or a methyl group; R2 is a hydrogen or a methyl group; and R3 is nitrile or chlorine or 
wherein, when c is 0, R4 is methyl, phenyl, or substituted phenyl, and when c is greater than 0, R4 is methyl, phenyl, substituted phenyl, halogen, nitrile or hydroxyl; m is greater than 1; n is greater than 1; b is greater than or equal to 0; and c is an integer from 0 to 38.
In one embodiment of the present invention, the acrylate polymer is soluble in an alkaline aqueous solution. In another embodiment, the acrylate polymer is a methylmethacrylate/ethylacrylate/methacrylic acid polymer, a methylmethacrylate/n-butylmethacrylate/methacrylic acid polymer, a methylmethacrylate/methylacrylate/methacrylic acid polymer, a methylmethacrylate/hydroxyethylmethacrylate/methacrylic acid polymer, a methylmethacrylate/hydroxypropylmethacrylate/methacrylic acid polymer, or a methylmethacrylate/propylacrylate/methacrylic acid polymer.
In another embodiment, an organic sensitizer is included with the acrylate polymer. Suitable sensitizers include, but are not limited to, polyaromatic waxes, methylbenzyl oxalates, for example, di-p-methylbenzyl oxalate, terphenyls, for example p-terphenyl, benzobiphenyls, fatty acids, fatty acid esters, for example, stearate, fatty amides, for example, stearamide, fatty acid salts, polyethylene waxes, dimethylphenyl ethane, for example, 1,2 bis(3,4-dimethylphenyl)ethane, carnauba waxes, microcrystalline waxes, and carboxy-modified paraffin waxes.
Optionally, the present invention may include a crosslinking agent. Preferably, the crosslinking agent is one that is suitable for use with polymers containing functional carboxylic acid groups. Suitable crosslinking agents include, for example, aziridine, epichlorohydrine, zinc oxide, and ammonium zirconium carbonate.
In yet another embodiment, a heat sensitive coating of the present invention may include a color former. Suitable color formers include fluorans, thiofluoranes, spiropyrans, triarylmethanes, xanthenes, and methynes.
In a further embodiment, the present invention may include a color developer. Suitable color developers include 4,4xe2x80x2-isopropylidenediphenol, 4,4xe2x80x3-diphenolsulfoxide, benzyl p-hydroxybenzoate, and phenol oligomers.
Optionally, the present invention may further include a filler selected from the group consisting of silica, zinc oxide, aluminum hydroxide, kaolin clay, talc, calcium carbonate, nylon powder, and polyethylene powder.
Another aspect of the present invention is a recording material which includes a heat sensitive coating of the present invention.
Yet another aspect of the present invention is a method of making an environmentally resistant heat sensitive recording which includes the following steps: (a) providing a heat sensitive coating comprising an acrylate polymer of the formula: 
wherein R1 is a hydrogen or a methyl group; R2 is a hydrogen or a methyl group; and R3 is nitrile or chlorine or 
wherein, when c is 0, R4 is methyl, phenyl, or substituted phenyl, and when c is greater than 0, R4 is methyl, phenyl, substituted phenyl, halogen, nitrile or hydroxyl; m is greater than 1; n is greater than 1; b is greater than or equal to 0; and c is an integer from 0 to 38; (b) coating a substrate with the heat sensitive coating; and (c) thermally activating the heat sensitive coating.
The invention will be understood further upon consideration of the following description.
An environmentally resistant heat sensitive coating has been developed. That is, the heat sensitive coating is resistant to water, oil, cleaning solutions and plasticizers, as well as other environmental stresses including scuffing and scratching, thus obviating the necessity and expense of a protective coating.
Generally, the invention disclosed herein includes an environmentally resistant heat sensitive coating including an acrylate polymer of the formula: 
wherein R1 is a hydrogen or a methyl group; R2 is a hydrogen or a methyl group; and R3 is nitrile or chlorine or 
wherein, when c is 0, R4 is methyl, phenyl, or substituted phenyl, and when c is greater than 0, R4 is methyl, phenyl, substituted phenyl, halogen, nitrile or hydroxyl; m is greater than 1; n is greater than 1; b is greater than or equal to 0; and c is an integer from 0to 38.
In one embodiment of the invention, the acrylate polymer is soluble in an alkaline aqueous solution. This solubility is due in part to the presence of carboxylic acid groups, which are neutralized in alkaline aqueous solutions. Without wishing to be bound to any particular theory, it is believed that the acrylate polymers of the present invention, being not readily soluble in neutral and acidic water, increase the resistance of the coating to water damage. Preferably, the acrylate polymer also has a glass transition temperature (Tg) equal to or greater than about 100xc2x0 C.
One polymer suitable for use in accordance with the present invention is methylmethacrylate/ethylacrylate/methacrylic acid. This polymer has a glass transition temperature (Tg) of about 100xc2x0 C., an average molecular weight of about 60,000, an acid number of about 120, and is soluble in an alkaline aqueous solution. This polymer is commercially available under the trade name Elvacite(trademark)-2669 from ICI Co. (Memphis, Tenn.).
Another suitable polymer is methylmethacrylate/n-butylmethacrylate/ methacrylic acid with a weight ratio of approximately 70/5/25 respectively. Other acrylate polymers suitable for use in accordance with the present invention include methylmethacrylate/methylacrylate/methacrylic acid polymers, methylmethacrylate/hydroxyethylmethacrylate/methacrylic acid polymers, methylmethacrylate/hydroxypropylmethacrylate/methacrylic acid polymers, and methylmethacrylate/propylacrylate/methacrylic acid polymers
Methods of making acrylate polymers by homogeneous free-radical polymerization such as bulk, solution and suspension polymerization are well known to those skilled in the art.
In another embodiment of the invention, the heat sensitive coating further includes an organic sensitizer. Without wishing to be bound to any particular theory, it is believed that when the heat sensitive coating is thermally activated to form an image, the acrylate polymer in the presence of the organic sensitizer produces a change in the coating which results in a further improvement in the resistance of the coating and recording material to water, oils and plasticizers.
Suitable organic sensitizers have a melting temperature of between about 50xc2x0 C. and about 200xc2x0 C. and are capable of facilitating the dissolution of the color former and the color developing material during the thermal activation of the heat sensitive layer.
Suitable sensitizers include, but are not limited to, polyaromatic waxes, methylbenzyl oxalates, for example, di-p-methylbenzyl oxalate, terphenyls, for example p-terphenyl, benzobiphenyls, fatty acids, fatty acid esters, for example, stearate, fatty amides, for example, stearamide, fatty acid salts, polyethylene waxes, dimethylphenyl ethane, for example, 1,2 bis(3,4-dimethylphenyl)ethane, carnauba waxes, microcrystalline waxes, and carboxy-modified paraffin waxes. Suitable stearamide is commercially available, for example, from Witco Co. (Greenwich, Conn.). 1,2 bis(3,4-dimethylphenyl)ethane, suitable for use in accordance with the present invention is available, for example, under the trade name ADK ARKLS Y-7 from Adeka (Tokyo, Japan). Similarly, suitable di-p-methylbenzyl oxalate is commercially available from DIC (Tokyo, Japan) and p-terphenyl is available from Aldrich (Milwaukee, Wis.).
The heat sensitive coating of the present invention may include a color former, including any leuco dye customarily employed in heat sensitive coatings. For example, fluoran, thiofluorane, spiropyran, triarylmethane, xanthene, and methyne, among others, are suitable for use with the present invention.
The heat sensitive coating may further include any developer used in heat sensitive coatings. Such developers are well known to those skilled in the art and include, for example, 4,4xe2x80x2-isopropylidenediphenol, 4,4xe2x80x2-diphenolsulfoxide, benzyl p-hydroxybenzoate, and phenol oligomers.
Also suitable for use with the invention are crosslinking agents. Preferably, the crosslinking agent is suitable for use with polymers containing functional carboxylic acid groups, such as, for example, aziridine, epichlorohydrine, zinc oxide, and ammonium zirconium carbonate.
The heat sensitive coating may further include various additives for heat sensitive coatings. For example, an organic or inorganic filler such as, for example, silica, zinc oxide, aluminum hydroxide, kaolin, clay, talc, calcium carbonate, nylon powder and polyethylene powder may be used in accordance with the invention. In particular, the addition of aluminum hydroxide and/or calcium carbonate is preferred as it improves the water resistance of the heat sensitive coating.
The invention also may include additional additives including, but not limited to, biocides, dispersants, wetting agents and defoamers. Such additives are typically application-specific and are well known in the art.
Suitable substrates for use with the invention include, but are not limited to, paper, cardboard, foam core, fabric, and plastic films like polyethylene terephthalate film, commonly known as Mylar, polyextruded photobase paper, and polyethylene film. Accordingly, suitable substrates may be opaque, translucent or clear.
The present invention also includes an environmentally resistant heat sensitive recording material that includes a substrate and a heat sensitive layer as described above.
Optionally, the recording material of the present invention may include further coatings including intermediate coatings disposed between the substrate and the heat sensitive coating, coatings disposed on the heat sensitive coating but not substantially contacting the substrate, and/or coatings disposed on the other side of the substrate opposite the heat sensitive coating. These optional coatings may be used for a variety of purposes, including, for example, to reduce curl, to create decorative effects and/or to create an adhesive layer for use as, for example, labels. Such coatings are well known in the art.
The heat sensitive coatings of the present invention generally exhibit improved adhesion to plastic substrates and other non-cellulosic substrates, thus obviating the need for an adhesion coating disposed between the plastic substrate and the heat sensitive coating. These intermediate adhesion coatings, sometimes referred to as tie-coats, typically are necessary with conventional coatings, especially for use with non-cellulosic substrates. Because the heat sensitive coatings of the present invention do not require an intermediate adhesive layer, cost savings as well as simplification of the process to produce the recording material of the present invention may be realized.
A heat sensitive coating of the present invention may be prepared by uniformly dispersing the components in water. The heat sensitive coating then may be applied to a substrate using a variety of techniques well known in the art for coating substrates, including, coating with a wire bar.
The recording material of the present invention is suitable for any conventional use for recording materials including tickets, for example, airline, railroad, concert and lottery tickets, and for labels, including supermarket and medical labels. Further, the heat sensitive coating and recording material of the present invention may be used with a variety of printers and processes, including, but not limited to, offset and flexo printing.